The present invention relates to an optical fiber array.
Due to the increasing density of optical fibers, more and more fibers are used for planar waveguides (PLC). To avoid the increase in size of waveguide elements associated with the increasing number of fibers and to further increase the density, efforts are being made to reduce the conventional standard type waveguide pitch (250 .mu.m) down to about a half, that is, 127 .mu.m.
To account for the increasing density of optical fibers and the decreasing waveguide pitch, efforts are also being made to reduce down to 127 .mu.m the inter-fiber pitch of fiber arrays connected to optical fibers.
As a conventional technique for reducing the pitch of fibers, a configuration has been proposed and examined wherein two sets of optical fibers 14 of standard tape fibers 13 are mutually overlapped in such a way that the upper and lower fibers 14 are alternately aligned in V-shaped grooves 16 in a V-shaped groove substrate 10 as shown in FIG. 7 (Document: see 1997 Electronic Information Communication Society Electronics Society Convention, C-3-113 "PLC Splitter Module Using 127 .mu.m-pitch Optical Fiber Array").
This configuration, however, requires taper to overlap as shown in FIG. 6, and unless the distance between the covered optical fibers 13 and the end of the substrate 11 where the naked fibers 14 are held is set at a predetermined value or more, this taper angle increases to cause losses or defects such as open circuits. Thus, to minimize the losses, the bend radius of the naked fibers 14 is set at 20 mm or less.
In the fiber array according to the above conventional document, the V-shaped grooves and a step portion 12 are formed in the V-shaped groove substrate 10 so as to mount covered optical fibers in the step portion as shown in FIG. 7.
In such a fiber array having such a V-shaped groove substrate 10, since normal tape fibers have a 250-.mu.m pitch, for example, the fiber interval at both ends of an 8-core tape fiber is 1.75 mm. Due to the manufacturing error of about 0.1 mm, when the covered optical fibers 13 are arranged in such a way as to contact the step portion 12, the naked fibers 14 are distinctly tapered to cause an open circuit. If the naked fibers 14 is shifted slightly backward from their original positions, the open circuit can be avioded because the taper is decreased. However, this requires increase in adhesive stress and adjustment. The fibers have to be shifted about 2 mm backward to secure a bend radius of 20 mm of the fibers. There is some apprehension that this cause deterioration in properties of the optical fibers due to the adhesive stress in this part.
FIG. 5 shows an example of a half-pitch fiber array in which the conventional pitch of 250 .mu.m is reduced to about a half.
In this figure, a covered fiber housing substrate 15 is stuck and fixed to a lower substrate (V-shaped groove substrate) 10 having V-shaped grooves, from above a step portion 12 in the V-shaped groove substrate 10. Then, tape fibers (covered fibers) 13a and 13b are inserted through a covered fiber housing groove 17 formed in the covered fiber housing substrate 15, in such a way as to overlap each other, and the upper and lower fibers are alternately aligned in the V-shaped grooves. Then, an upper substrate (a fiber presser substrate) 11 is installed and fixed from above the V-shaped grooves in the V-shaped groove substrate 10 to form an optical fiber array 22.
In FIG. 5, to reduce the taper angle of the overlapping fibers, the fibers are shifted slightly backward from their original positions.
Since, however, an adhesive is filled in the overlapping fiber portion 21, the thermal expansion of the adhesive causes stress in the fibers to degrade the characteristics of the optical fiber array 22, for example, to cause losses in the optical fiber array.
In addition, shifting the fibers slightly backward requires adjustments, and shifting them backward, for example, about 4.9 mm enables a sufficient bend radius of the fibers to be obtained to avoid losses. If, however, adjustments are insufficient and the shifting distance is short, the bend radius of the fibers decreases to increase the taper angle, while if the shifting distance is excessively long, the amount of adhesive filled in the overlapping fiber portion increases possibility to degrade the characteristics of the optical fiber array 22.